The provision of circuit breakers in direct current transmission systems has during a long time been a serious challenge because of the lack of current zero crossings in the DC current.
One way of addressing this problem is through the use of a hybrid circuit breaker. One example of this circuit breaker is the hybrid circuit breaker described in WO 2011/057675. The circuit breaker comprises at least one main circuit breaking block comprising a branch with a number of series-connected power semiconductor switches. In parallel with this branch there is a non-linear resistor, such as a varistor. The main circuit breaker may then be made up of a series of such main circuit breaker blocks. In parallel with the main circuit breaker blocks there is a series-connection of an ultrafast mechanical disconnector and a load commutation switch, which load commutation switch is also made up of a number of series-connected power semi-conductor devices. The ultrafast mechanical disconnector and load commutation switch together make up a transfer switch.
This type of hybrid circuit breaker has the following operation:
When the circuit breaker is to be opened, the load commutation switch first opens, which commutates the current to the main circuit breaker. Thereafter the ultrafast mechanical disconnector is opened for mechanically disconnecting the load commutation switch. Finally the main circuit breaker is opened, which commutates the current into the non-linear resistor. It is then also possible to use standard type or conventional disconnectors to mechanically separate the hybrid circuit breaker from the DC system.
In WO 2011/057675 every hybrid circuit breaker comprises such a transfer switch, which thus has the purpose of directing the current through the main circuit breaker.
In DC transmission systems it is nowadays also of interest to interconnect a number of DC networks via a switchyard.
In a development of this hybrid circuit breaker, the transfer switch is removed from the hybrid circuit breaker. When this is done it is possible to use only one main circuit breaker in the switchyard interconnecting a number of DC networks, where a number of transfer switches are used for directing current through this main circuit breaker. An example of the use of a single main circuit breaker for connection of three or four DC networks is disclosed in WO 2013/068046.
The document also discloses the use of two main circuit breakers in switchyard based on the use of two busbars. The provision of two main circuit breakers seems to be made in order to provide redundancy in case maintenance is required.
However, there is still room for improvement in the provision of a switchyard using more than one main circuit breaker and a number of transfer switches. There is more particularly a need for providing a structure that is easily scalable for adding more main circuit breakers and more DC network connections to a switchyard.